单项选择题

Vitamins are complex ______ that the body requires in very small mounts.

A．matters
B．materials
C．particles
D．substances
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单项选择题
Not having a good command of English can be a serious ______ preventing you from achieving your goals.
A．obstacle
B．fault
C．offense
D．distress
单项选择题
John doesn t believe in ______ medicine; he has some remedies of his own.
A．standard
B．regular
C．routine
D．conventional
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